1. Field of the Invention
This invention relates to a digital signal processor for processing audio digital signals to perform equalizing or adding of reverberate sound, and a control method for changeover of the sampling frequency of the digital signal.
2. Description of the Related Art
A conventional digital signal processor is capable of changeover and processing of several kinds of audio digital signals (hereinafter referred to simply as digital signal) each having a different source and is used in a system shown in FIG. 1. Referring to FIG. 1, digital signal from an A/D converter 1 for converting analog data to digital data, digital audio interface receiver (DIR) 2 for outputting optical signal from a mini disc (MD) player in the form of a digital signal, CDLSI 3 for outputting a digital signal read from a compact disc (CD), a DVDLSI 14 for outputting a digital signal read from a digital video disc (DVD) or the like is input to a digital signal processor 6 through a selector circuit 5 as a source of object digital signal for processing.
A microprocessor 7 controls the selector circuit 5 so as to change over the selector circuit 5 so that a digital signal corresponding to a source selected by user's key operation is input to the digital signal processor 6. At the same time, the microprocessor (MCU) 7 sets a processing program and processing data such as coefficient data, offset data, and the like corresponding to a sampling frequency of a selected digital signal in the digital signal processor 6.
The digital signal processor 6 is supplied with left/right identification clock LRCK for identifying left and right channels of an audio signal and bit clock BCK for sending digital signal DATA as well as digital signal DATA which is object audio data for processing. Usually, a sampling frequency of an object digital signal for processing is specified by the left/right identification clock LRCK so that the digital signal processor 6 carries out processing of input digital signal DATA smoothly.
After the input digital signal is subjected to equalizing, addition of reverberate sound, surrounding processing or the like, the digital signal processor 6 outputs to the D/A converter 8. The D/A converter 8 converts the input digital signal DATA to analog data and then outputs to an audio amplifier (not shown) or the like on a next stage.
If the kind of a source of an object digital signal for processing is changed over by user's key operation (not shown), the microprocessor 7 sets a corresponding processing program or processing data in the digital signal processor 6, which carries out processing of a source selected by the user.
Here, because a processing program for processing digital signal DATA which is operated inside the digital signal processor 6 carries out processing synchronous with the sampling frequency, the processing program and processing data such as coefficient data and offset data must be changed over each time when the sampling frequency of an object digital signal DATA for processing is changed over.
Particularly, in case of a digital filter operation and data delay processing using memory, the processing data (coefficient data, delay memory offset data and the like) differs depending on each sampling frequency. Therefore, the changeover of the processing data by the microprocessor 7 corresponding to a changeover of the sampling frequency is a very important processing.
When the sampling frequency of digital signal DATA of an object for processing is changed over during program processing by the digital signal processor 6, the program processing action becomes unstable, so that a processed digital signal DATA becomes unstable. Thus, in case processing program or processing data for the digital signal processor 6 is replaced by the microprocessor 7, output stop of processed digital signal DATA or operation stop of the processing program in the apparatus also must be controlled from the outside by the microprocessor 7.
In a system having to make the digital signal processor 6 carry out processing of a great deal of processing program and processing data, a burden of the control program of the microprocessor 7 increases, so that halt time of output signal during a replacement control of the aforementioned processing program or processing data is prolonged and further, the other control by the microprocessor 7 get into trouble.
In case the sampling frequency deviates from a predetermined frequency or the frequency becomes zero, disabling any input, this is called noise in the sampling frequency. When this noise occurs also, such a control as an operation stop of an operation processing circuit in the digital signal processor 6 is executed so as to prevent any uncomfortable sound from being output outside by the microprocessor 7. This control also gives a burden on the microprocessor 7, thereby producing the same problem as described above.
Although, to avoid this problem, use of a high speed microprocessor 7, expansion of work memory of the microprocessor 7, adding of a mask ROM or the like is necessary, these countermeasures induce an increase of system production cost.